1. Field of the Invention
The present invention relates generally to the heat treating process known as annealing, and, more particularly, to a convection diffuser and charge support system for use in an annealing furnace, and to methods for use in forming selected components of the system from nodular cast iron such that they include cast-in-situ cooling conduits.
2. Prior Art
Annealing is a heat treatment process whereby a charge of material is heated to and held at an elevated temperature for a sufficient length of time to assure that metastable conditions in the material, such as frozen-in strains, dislocations, vacancies, and the like are permitted to achieve thermodynamic equilibrium. With ferrous materials, the term "annealing" is usually used in the sense of a "full" annealing process which involves a change of phase, whereby the metal is heated into the austentic region, and thereafter cooled back to ambient temperature to develop a softened structure of pearlite and ferrite within the metal.
Where the charge of ferrous material being annealed has been cold-worked, the annealing process is used to soften the material to relieve such hardness as has been induced during cold working. Cold-working tends to increase the dislocation density of a metal manyfold. By way of example, a cold-worked piece of metal may have a dislocation density that is 10.sup.6 greater than that of an unworked specimen of the same material. Since dislocations within cold-worked metal are surrounded by strain fields, the greater the number of dislocations, the greater is the magnitude of the "free energy" which is stored in these strain fields and which can be released during annealing to furnish a driving force that will assist in bringing the dislocation density back to within a desired range.
In order to properly anneal a charge of ferrous material, it is important to confine the charge within an enclosure wherein a non-oxidizing environment is maintained. The gases which define the non-oxidizing environment must be circulated within the enclosure during annealing to assure that convection heat transfer takes place efficiently to enable the annealing process to be carried out in a reasonable period of time. Similarly, during cooling of the charge, proper gas flow is important to effect convection cooling.
Where the charge of material to be annealed takes the form of a plurality of coils of rolled steel, the enclosure utilized to surround and support the charge conventionally includes an annular base structure atop which a vertical stack of the coils of steel to be annealed is supported, with the coils positioned coaxially one atop another. The enclosure also includes a generally cylindrical shroud which cooperates with the base structure to contain the stacked charge of coils and to define an enclosed environment within which hot gases of the controlled environment are caused to circulate.
In conventional practice, a fan is disposed in a centrally-located chamber or hole formed through the base structure for forcing circulation of the gases of the non-oxidizing atmosphere throughout the enclosure. The conventional flow path for circulation includes a flow of gases downwardly through the stack of coils, and upwardly along the outer surfaces of the coils. Convector plates are interposed between adjacent ones of the stacked coils to provide convection flow paths for diverting some of the circulating gases between the ends of adjacent coils. The base structure on which the lowermost coil rests is provided with vanes for directing gases discharged from the fan outwardly and upwardly about the stack of coils.
While the gas circulation passages of a newly-built diffuser base may provide a gas flow pattern that is relatively effective in disbursing gases throughout the enclosure, once the newly-built base has been in service for several months, its flow passages often become deformed due to thermodynamically induced stress which results in creep growth that requires trimming, with the result that the passages no longer operate as intended to properly direct gas flow.
Moreover, inasmuch as the structures which define the vanes of present-day diffuser bases are traditionally formed as weldments of relatively soft steel, the vanes tend to become deformed and/or broken during use, thereby further adding to the inefficiency and unpredictability of a diffuser base after it has been in use for a significant period of time. Thus, present day diffuser bases not only fail to operate efficiently and effectively over long periods of time, but also require frequent checking for structural integrity, cleaning and repair.